Diagnosis of materials and effectiveness of Er:YAG laser cleaning as complementary treatment in a panel painting attributed to Lluís Borrassà (fifteenth century)

(English) A fifteenth-century panel painting by Lluís Borrassà (1360-1425) was the object of a restoration at the North Carolina Museum of Art. A multi-analytical approach based on Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy and gas chromatography-mass spectrometry (GC/MS) also coupled with a pyrolyzer was used to investigate the origin of the darkened coating obscuring the work of art in order to find a proper cleaning procedure. The results evidenced the presence of different restoration materials that constituted the superficial patina, which was difficult to remove by conventional solvent cleaning methods. The Er:YAG laser operating at 2.94 μm was tested as an innovative alternative method to the traditional solvent mixture and gels. Because of the presence of multilayers of over-painting and restoration materials, the Er:YAG laser also proved beneficial as a nondestructive and selective sampling tool. The organic material was collected by positioning a microscope glass coverslip (15 × 15 mm) on the painting surface, followed with laser ablation that removed the material, which condensed on the glass that was then submitted for the FTIR and GC-MS analyses. The acquired new knowledge of the characterized materials and optimization of the cleaning methods guided the conservators to choose the best restoration conditions and perform a safe and successful cleaning of the painting especially the blue pigment, which was covered with intractable restoration material. The Er:YAG was an efficient and safe method to remove these materials.